In the manufacture of plastic book covers, it is important to control the depth and uniformity of the hinge lines. To do this mechanically poses some problems:
1. To get all of the surfaces of a dielectric die in exactly the same plane requires skillful machining and maintenance.
2. To keep the aforementioned plane of the die and the plane of the bed plate of the press exactly parallel requires skillful set-up, continual checking and better than average (and therefore more expensive than average) equipment.
Uniformity of hinge lines can be more easily obtained by the apparatus of this invention which uses a laminated material for the book covers with each component of the laminate different from the other. One component is responsive to electronic heating (also known as dielectric or radio-frequency or RF heating) and the other component has little or no response to the high frequency electric field.
Materials which are heated to a softened condition and then to a fused condition are commonly referred to as "dielectrically heat sealable." They can be sealed in a surface of the same material or to another heated material by being exposed to an alternating electro-magnetic field of high frequency with the surfaces of the material in contact with one another. Polyvinyl chloride (often referred to as PVC) is one of the best known dielectrically heat sealable materials.
Heat sealable plastics are suitable for this invention, but the invention only requires sufficient heat to soften the plastic to a flowable condition, so any material which can be dielectrically heated to a flowable condition is suitable for this invention even though it does not fuse. In fact, the preferred embodiment of the invention stops the heating when the material reaches a condition where it will flow under pressure but is not sufficiently hot to fuse or even to flow back into a groove from which it was displaced by pressure. For purposes of this invention, plastic material that can be heated to a flowable condition by dielectric heating will be referred to as "dielectrically heat sealable."
Book covers made in accordance with this invention are laminated. This means that the cover is made of at least two layers of plastic which are bonded together, preferably over the entire areas of their interface. One layer is made of heat sealable material and the other layer is made of plastic material that is less highly heated or not heated to any consequential extent by a high frequency electric field.
Theoretically, every material is probably heated to some extent by an alternating electric magnetic field, since only a vacuum has perfect loss characteristics; but for purposes of this invention it is sufficient that the second layer be heated less than the first layer and that the second layer remain at a temperature so low that it does not soften enough to undergo permanent displacement when subjected to the tool pressure that is used to displace the material along the hinge line of the first layer. For purposes of this specification, this lesser degree of heating of the second layer will be referred to as "inconsequential heating." Polypropylene is a well-known example of a plastic that undergoes only inconsequential heating in an alternating high frequency electromagnetic field.
This invention makes books with the filler of the book confronting the grooved front and back panels, or vice-versa, depending upon the desired style of the book.
The covers of this invention are preferably formed from a strip of laminate that moves continuously through a hinge forming station; and that is cut to form book covers at a shearing or cut-off station; but the invention can be used with batch operation when continuous facilities are not available. The laminates can be in the form of a web or sheet.
In addition to providing control of depth and uniformity of hinge lines, this invention has other advantages in making plastic book covers. For example, the laminate can be made by bonding together sheets of different colors in order to obtain unusual and decorative effects. Also, the laminated sheet can be fabricated with the layer that is not to be grooved made of more flexible material particularly well suited for repeated bending as a hinge, and the heat sealable layer can be made of less flexible material for imparting stiffness to the cover over areas other than the hinge lines. The material that does not have to provide the hinge areas can be made of less expensive plastic to reduce the total cost of the cover.
Another feature relates to the forming of hinge lines by means with adjustable spacing of the hinge lines so that the same apparatus can make covers for books of different thickness. Other means adjustable to cut the covers to different widths compensate for the wider or narrower spine area of the cover between the hinge lines.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.